Do hard plastic insoles cause more damage to the plantar fascia?

I have never heard such a thing because it is supposed to heal your pain and it might depend on a lot of factors. Hard insoles don't do more damage to the plantar fascia, but soft ones are comparatively more effective and more comfortable to wear. But hard insoles are sturdy, so it is not always easy to wear them.

It boils down to exactly what you need. While hard insoles are great for support and control, soft insoles offer support and flexibility. In the case of plantar fasciitis, we recommend a semi-rigid orthotic that combines the effect of the hard insole with the advantage of the soft insole.